Transcript Cladding

Fiber Optics
Introduction
Optical fiber is a long thin transparent dielectric material
which carries EM waves of visible and IR frequencies
from one end to the other end of the fiber by means of
TIR.
NOTE: Glass or Plastic is used as Dielectric material.
Optical fibers works as Wave guides in optical television
signals, digital data to transmit voice television signals,
digital data to any desired distance from one end to the
other end of the fiber.
Optical fiber consists of three sections
1. Core
2. Cladding 3. Protective Jacket
Core: It is an inner cylindrical material made up of glass or plastic.
Cladding: It is a cylindrical shell of glass or plastic material in which
Core is inserted.
Protective Jacket: The Cladding is enclosed in polyurethane jacket and
it protects the fiber from surroundings.
NOTE: The RI of core is slightly greater than the RI of Cladding. The
normal standard values are 1.48 and 1.46 respectively.
Structure of an Optical fiber
Poly urethane protective jacket
Cladding
Core
• Principle: Optical fiber works on the principle of TIR. Once
light ray enters into core ,it propagates by means of multiple
TIR’ s at core-cladding interface.
RERAR MEDIUM
NORMAL
NORMAL
r
r = 90


DENSAR MEDIUM
  c
  c
NORMAL
RERAR MEDIUM

DENSAR MEDIUM
  c
according
to law of refraction
n1 sin   n 2 sin r
   c  r  90
sin  c 
n2
sin  c 
n2
n1
n1
0
sin 90
0
Acceptance Angle
• The maximum angle of incidence at the end face of
an Optical fiber for which the light ray can be
propagated along Core-Cladding interface is known
as maximum Acceptance angle. It is also called
Acceptance cone half angle.
Core-Cladding interface
B
θr
θ
Core n1
θr
θi A
C
Fiber axis
Cladding n2
Incident light ray
Applying Snell’ s law for Air-Core media
n 0 sin  i  n1 sin  r .......... ....( 1)
from the right angle triangle
 r    90
0
 r  90  
0
n 0 sin  i  n1 sin( 90   )
0
n 0 sin  i  n1 cos 
sin  i 
n1
n0
cos  .........( 2 )
ABC
when   critical
n1
sin  m 
n0
according
angle ( c )   i   m
cos  c .......... ......( 3 )
to law of refraction
n1 sin i  n 2 sin r
i   c  r  90
sin  c 
n2
sin  c 
n2
cos  c 
sin 90
0
n1
n1
1  sin
2
cos  c 
0
2
n1  n 2
n1
c 
1 (
n2
n1
2
.......... ..( 4 )
)
2
substitute
sin  m 
equation
2
n1
n1  n 2
n0
n1
if the medium
sin  m 
 max  sin
2
surroundin g the fiber is air , then n 0  1
2
n1  n 2
1
( 4 ) in ( 3 )
2
2
n1  n 2
2
Which is required expression for
Acceptance Angle in optical fibers.
Maximum
Acceptance Cone
Rotating the Acceptance angle about the fiber axis
describes the Acceptance Cone of the fiber.
Light launched at the fiber end within this Acceptance
Cone alone will be accepted and propagated to the other
end of the fiber by total internal reflection.
θm
θm
Acceptance Cone
Numerical Aperture
• The light gathering capacity of an
optical fiber is known as Numerical
Aperture and it is proportional to
Acceptance Angle.
• It is numerically equal to sine of
minimum Acceptance Angle.
NA  sin  max
2
n1  n 2
sin  max 
2
n0
2
2
NA 
n1  n 2
NA 
( n1  n 2 )( n1  n 2 )
 
n1  n 2
n1
NA 
The ratio between the difference
in RI’s of
Core and Cladding to that of RI of
core is called the fractional change.
n1  ( n1  n 2 )
n1  n 2
NA 
NA  n1
2
n1 2 
2
TYPES OF OPTICAL FIBRES
On the basis of variation of RI of core, the optical fibers
are mainly classified into following types. i.e.,
1.Step Index fiber 2.Gradex Index fiber
NOTE: Based on Mode of propagation, the fibers are
further divided into Single Mode and Multi Mode.
Single Mode Step Index fiber
The RI is constant for the core in this fiber. As we go radically from center
of the core, the RI undergoes a step change at core-cladding interface .
The core diameter of this fiber is about 8 to 10µm and the outer diameter
of cladding is 60 to 70µm.
There is only one path for light ray propagation. Hence it is called single
mode step index fiber.
It is a reflective fiber since light is transmitted from one end to the other
end of a fiber by TIR.
These are extensively used because distortion and transmission losses are
very less.
Refractive index profile of
single mode step index fiber
60 to 70 µm
8 to 10 µm
RI
Radial distance
SINGLE MODE STEP INDEX FIBER
CORE
RAY
PROPAGATION
CLADDING
Multimode Step Index Fiber
The construction of this fiber is similar to Single mode step
index fiber but dimensions of Core and Cladding are much
larger to have more number of paths for light propagation.
The Core diameter varies from 50 to 200µm and the
Cladding diameter varies from 100 to 250µm.
It is also a reflective fiber since light is propagated in the
form of multiple TIRS.
REFRACTIVE INDEX PROFILE OF MULTI MODE STEP INDEX FIBRE
100 to 250 µm
50 to 200 µm
RI
Radial distance
GRADED INDEX FIBRE
In this fiber , Radially the RI of Core continuously decreases
from center to the surface.
The RI is maximum at the center of Core and Minimum at
the Surface.
This fiber can be a single mode or Multimode ,the diameters
of core and cladding varies from 50-200µm and 100-250µm
respectively.
REFRACTIVE INDEX PROFILE OF SINGLE MODE GRADED INDEX FIBER
100 to 2500 µm
50 to 200 µm
RI
Radial distnce
REFRACTIVE INDEX PROFILE OF MULTIMODE GRADED INDEX FIBRE
100 to 250 µm
50 to 200 µm
RI
Radial distance
• As RI changes continuously radially in Core, the light rays
suffers continuous refraction with in the Core from its center
to surface.
• Thus the propagation of light rays are not due to TIR but
by refraction. Therefore it is called Refractive fiber.
• In this fiber, the light rays travel at different speeds in
different parts.
• Near the surface RI is least so, the light rays travel faster
compared to the light rays near the axis. Because of this all
the rays almost arrive at the same time at the other end of the
fiber.
LIGHT PROPAGATION IN MUTI-MODE
GRADED INDEX FIBRE
CLADDING
CORE
Optical fiber Communication System
• An efficient optical fiber communication system
requires high information carrying capacity such
as voice signals, video signals over long
distances with a minimum number of repeaters.
It essentially consists of following parts.
1.Encoder
2. Transmitter
4.Receiver
5.Decoder
3.Wave guide
• 1.Encoder: It converts electric signal corresponding to
analog information such as voice, figures, objects etc
into a binary data. This binary data comes out in the form
of stream of electrical pulses.
• 2.TRANSMITTER: It mainly consists of drive circuit and
a light source. Drive circuit supplies the electric pulses to
the light source from the encoder.
• NOTE: LED or diode laser is used as light source and it
converts electrical signals are infected into optical
signals. These optical signals are injected into wave
guide.
TRANSMITTER
OPTICAL SIGNAL
Electrical
signal
DRIVE SOURCE
LIGHT SOURCE
ENCODER
PHOTO
DETECTOR
SIGNAL
RESTORER
Wave guide
AMPLIFIER
receiver
DECODER
Attenuation or Power loss in Optical fibers
• The power of the light at the out put end is found to be
always less than the power launched at the input end.
• Attenuation is found t be a function of fiber material,
wavelength of light and length of the fiber and it is
measured in terms of the decibel.
Attenuation mainly three types….
1.Scattering losses
2.Absorption losses
3.Bending losses